Pretensioners are used to rapidly pull in slack in a safety restraint seat belt at the onset of a crash to more securely restrain the vehicle occupant against forward movement and potential injury by collision with structural features of the vehicle. In addition, the pretensioning operation pulls the occupant into, or at least towards, the correct seating position to maximize the effect of a second restraint such as an airbag.
A modern seat belt is known as a 3-point restraint because it is secured to the vehicle at three points arranged about the vehicle occupant to provide a diagonal torso section and a horizontal lap portion to hold the vehicle occupant in the seat. The belt is attached to the vehicle by a spring-loaded retractor tending to tighten in the belt, and by a buckle for quick release of the belt.
Pretensioners are typically sited at the retractor or at the buckle end of the restraining seat belt. They are designed to store energy which when released effects the pretensioning operation. This energy may be mechanical energy in the form of a stressed spring, but more modern pretensioners are pyrotechnically operated. Pyrotechnically operated pretensioners comprise a sealed tube containing a gas generant composition which reacts rapidly to generate gas which expands rapidly to provide the energy to effect the pretensioning operation. The gas generant composition is ignited electrically via pin terminals protruding from the pretensioning tube and connected to an intermediate igniter material.